1. Field of the Invention
This invention relates generally to optical telecommunication systems and, more particularly, to optical receivers configured to receive differential optical signals.
2. Description of the Related Art
In an effort to meet increasing capacity needs associated with wavelength-division multiplexed (WDM) optical telecommunication systems, various modulation schemes have been developed. A receiver configured to demodulate differential modulation schemes, such as differential phase-shift keying (DPSK) and differential quadrature phase-shift keying (DQPSK) signals, may be configured to determine the change in phase of a received data signal over time, as opposed to the determination of the actual phase of the data signal with reference to a local oscillator, associated with other phase-shift keying schemes. The changes in phase over time in the received data signal thus represent the actual data encoded in the data signal. Such systems may rely on observing a phase change between a received data signal and a reference signal directed derived from the received signal, the reference signal being equal to the received data signal, however, delayed by one bit period for example. The phase change between the received data signal and the reference signal thus representing the encoded data. The phase difference between the received data signal and the reference signal would then represent the encoded data, which can be visualized on a constellation diagram for example, as is well known in the art.
Such differential modulation schemes, such as differential phase-shift keying (DPSK) and differential quadrature phase-shift keying (DQPSK), can be significantly easier to implement in optical receivers since a local oscillator is not necessarily needed at the receiver to determine a phase change with respect to a received data signal. However, the signal-to-noise ratio (SNR) performance of such a differential modulation scheme such as DPSK or DQPSK may be generally worse than coherent schemes, which may rely on a local oscillator at the receiver for example. One source of noise in a receiver configured to implement differential modulation schemes may be optical noise on the delayed reference signal utilized in the demodulation. For example, optical noise on the delayed reference signal, may lead to corresponding phase errors in the delayed reference signal in the optical domain. Once demodulated, these phase error may lead to corresponding errors in the demodulated signal. Since the phase errors generally impact two adjacent symbols, as part of a differential signal for example, and ultimately adjacent demodulated bits, these phase errors may be referred to as correlated phase errors or correlated errors between said two adjacent symbols. More information regarding differential receivers, among others, can be found in U.S. patent application Ser. No. 12/572,179, incorporated herein by reference in its entirety.
What is needed is an optical receiver having an increased tolerance to optical noise resulting in an optical line system having improved reach.